PURPOSE: To prevent the rust on the cut surface of a core by forming engaging recesses and projecting strips on the peripheral surfaces of the cut surfaces of a pair of cut cores, thereby effectively securing the airtightness between the cores.

CONSTITUTION: Lower cut cores 5, 6 and upper cut cores 36, 27 to be associated are molded in molding members 4, 26 made of insulator having weather resistance. The cut surface of the cut cores 5,6,36,37 are exposed on the surfaces of the molding members 4, 26, and engaging recesses and projecting strips 34 are formed on the peripheral surface of the respective cut surfaces at the time of associating the cores 5, 6, 36,27.